In the hunting and sport shooting fields, several types of shotguns are known which differentiate for the technical features and/or the functional solutions which are adopted.
The gauge is a parameter indicative of the barrel inner diameter. According to conventional system for hunting, the gauge is indicated by a number (mostly 12 or 20, more rarely 28), which indicates how many lead balls with the diameter of the barrel are contained in one pound.
The chamber length is expressed in inches. It indicates the maximum length of the cartridge which can be contained in the chamber. The most common chamber lengths are 2¾″ (corresponding to 70 mm, also called Standard), 3″ (corresponding to 76 mm, also called Magnum), and 3½″ (corresponding to 89 mm, also called Supermagnum). The more is the cartridge length, the higher is the amount of pellets that it may contain. To a higher amount of pellets corresponds a higher amount of powder and, consequently, a higher firing power.
Other characteristics which the different types of shotguns are distinguished from are the weapon general configuration, with fixed or tilting barrel (break-action); the barrel length, typically ranging between 560 mm and 800 mm; and the mouth choke which determines the distribution of the pattern of the shot on the target.
Furthermore, in the hunting and sport shooting fields, alongside the manual loading shotguns, semi-automatic shotguns have long been known. Such shotguns relieve the user of the obligation of necessarily manually loading the weapon. In fact, in the step which immediately follows each firing, the semi-automatic shotgun autonomously proceeds to the recocking of the mobile masses (slide-bolt assembly), the ejection of the hollow shell, and the feeding and chambering of the new ammunition.
Several types of semi-automatic shotguns are known, the operation of which is based on different principles.
A first type of semi-automatic shotgun is that called “gas-operated”. In such shotgun, the energy of the gases developed by the powder explosion is exploited. A small part of such gases is drawn from the barrel through one or more holes, in order to generate an expansion inside a cylinder closed by a sliding piston. The piston thrust generates, in turn, an impulse which recocks the mobile masses, ejects the shell, and loads the new ammunition.
The piston thrust is extremely variable as a function of the force of the primary impulse generated in the chamber by the powder explosion. Such primary impulse depends on the gram weight of the cartridge which is fired, where “gram weight” means the mass of the charge of fired pellets, therefore the power of the same cartridge. The mass of the powder charge and the mass of the pellets charge are typically proportional.
The gas-operated device, in order to be able to ensure the required reliability, must necessarily be dimensioned for the operation with those cartridges having the lowest gram weight which can be chambered in the shotgun. Once the device has been properly dimensioned, the variability of the cartridges gram weight and the consequent primary impulse translate in a variability of the recocking speed. The minimum speed is the one which is necessary in order to achieve a safe operation of the weapon when a cartridge having a low gram weight is fired. The maximum speed corresponds to the firing of a cartridge having the maximum gram weight which can be chambered in the shotgun.
However, the high recocking speeds translate in high stresses and, consequently, in a decrease of the working life of the shotgun components. In the whole, this results in a short duration of the same shotgun.
In the more modern gas-operated shotguns, it has been successfully attempted to obviate the problem of the high recocking speeds by adopting shutter or self-compensating valves, which are able to exhaust the excess gas associated to the firing of the cartridges having a higher gram weight.
However, such valves, or venting systems, involve an increase of the mechanics and the costs for the shotgun.
Furthermore, the gas-operated systems require a constant maintenance, since the gas which is vented tends to foul unburnt solids, which have to be removed after firing a number of shots.
Another type of semi-automatic shotgun is the one called the “inertial” type. In this type of shotgun, the compression and the subsequent relief of a spring that is arranged between the mobile masses and the shotgun frame are exploited. The spring compression is caused by the shotgun recoil, and it is exploited in order to confer to the same masses the required recocking speed.
The shotgun with inertial operation is appreciated because it allows limiting the maximum recocking speeds and the resulting reduction of the stresses of the mechanical parts.
Furthermore, the inertial shotgun is characterized by a pronounced constructive simplicity and a reduced maintenance of use. In fact, not requiring any gas drawing, the inertial device does not undergo any “fouling”. Anyway, the standard cleaning is still necessary for the chamber and the barrel, which are contacted by the firing gas.
In contrast, the low recocking speed, which is intrinsic of the inertial shotgun, may be a problem, especially when the shotgun frame has a high mass, and the fired cartridge has a low or very low gram weight. The low recocking speed translates in a low shell ejection promptness and a high risk of jamming.
Furthermore, the operation of such type of shotgun is highly affected by the user's behavior, particularly by the type of reaction which the user opposes with his/her shoulder to the shotgun stock.
A further type of semi-automatic shotgun, historically the first to be developed, is that called “barrel long recoil” type or, more simply, “long recoil”. In such type of shotgun, the natural recoil exerted by the gas thrust is exploited in order to backwardly accelerate the barrel and the slide-bolt assembly therewith, and all the masses involved in the recocking movement. Suitable unlock devices located between the barrel and the bolt provide to disconnect, at the right moment, the barrel from the locking members. The right moment to disconnect the barrel from the locking members is somewhat delayed compared to the moment when the shot leaves the muzzle and, as a result, the pressure inside the barrel is drastically decreased. Thereafter, a return spring brings the barrel back to the initial position, (called the battery position), while the slide-bolt assembly, provided with its own return spring, provides for the operations of shell ejection and reloading of a new ammunition.
In the long recoil shotgun, while awaiting the coming out of the shot from the muzzle, all the impulse of the gases in chamber is used in order to accelerate the barrel and the mobile masses. In fact, their recoil motion under the action of the gases extends during the whole recocking stroke, that is for many tens of millimeters. Incidentally, the recocking stroke must be approximately as long as the cartridge length, therefore ranging between 70 mm and 89 mm.
The exploitation of all the gases impulse in the chamber translates, in the case of the firing of cartridges having a high gram weight, in high recocking speeds. The high recocking speeds involve an elevated shaking of the shotgun, a high stress of the mechanical parts, and a high recoil for the user's shoulder.
In order to minimize the adverse effects of the long recoil, several devices have been proposed. Among these, for example, the friction brakes, to be actuated only in the case of the firing of Magnum or Supermagnum cartridges. Such devices, beside having a quite poor operational reliability, force the user to an additional burden, consisting in having to set the weapon as a function of the cartridge which is fired from time to time.